Caitlyn Jenner used to steal makeup from her youngest daughters so she could feel like a true woman, she's revealed in her new book.

The reality star, who up until April 2015 was living as Olympic gold medallist Bruce Jenner, has spoken candidly about how her male body left her with "loathing and disgust" and how she used to dress as a woman by sneaking dresses and wigs through airport security.

In an extract from her book The Secrets Of My Life, run by the Mail On Sunday's You magazine, Caitlyn admits how she was almost caught wearing women's underwear by a security guard who noticed the metal detector had gone off and prepared to search her.

Caitlyn has been living publicly as a woman for two years (Image: Twitter/@ABC2020)

"I braced myself for being marched to a private room to remove my shirt. The fear was palpable until it turned out the wand had picked up a zipper on my jacket. I was more careful after that," she writes.

Caitlyn, now 67, used to pilfer clothes from Kris Jenner without her knowing, and took makeup from Kourtney, Kim, Khloe, "and eventually Kendall and Kylie" because there was so much of it in the house.

"Applying makeup is always intense and I sometimes think I work harder on it than I did to win the decathlon," she says.

Caitlyn was forced to keep her clothes in a locked closet to prevent her children and step-kids from finding them (Image: E!)

Caitlyn feared being discovered wearing women's underwear in her old life as Bruce (Image: Splash News)

The then Bruce had been married to two other women prior to Kris – Chrystie Crownover and Linda Thompson – and said the reaction of all wives to her trans secret was similar.

"My wife Kris will not permit any of this behaviour [dressing as a woman] at home, just like my two ex-wives. She doesn't want to see it or deal with it, so we never talk about it," Caitlyn adds.

To prevent her five daughters and son Rob from finding out, Kris banned Caitlyn from having her clothes and makeup out on display – meaning Caitlyn had to put her possessions in a small, locked closet in the back of her wardrobe.

On front cover of Vanity Fair - her first public appearance as Caitlyn (Image: Annie Leibovitz)

"Kris and I have negotiated this, since she is terrified, as I am, of the kids finding something."

Eventually, keeping it to herself became too much for Caitlyn – who thought she might have to stipulate in her will to be buried as a woman – and she came out as trans.

"It is so easy, so natural, so effortless, so much fun: the way my life was always supposed to be," she adds.